This is a domestic postcard-rate stamp (currently 35c), and has a link to this whole A-Z series in that it depicts a seashell called an alphabet cone, Latin name Conus spurious. The common name results from the fact that the markings often look like letters, and may seem to spell out short words. Take care when handling a live one: they have a modified tooth that functions as a poisonous harpoon!
The only time I thought an Xed out stamp worked, was on this zombie envelope (not for the squeamish).
To see more postage with an X theme, head over to the links at Sunday Stamps.